1. Field of the Invention
The present invention relates to characterization techniques for transistor designs, and in particular, to methods for identifying safe operating areas (SOAs) for bipolar junction transistors (BJTs).
2. Description of the Related Art
High-speed bipolar transistor circuit designs have been implementing the BJTs as silicon-on-insulator (SOI) devices so as to reduce parasitic effects and improving packing densities. However, compared to conventional bulk silicon BJTs, such implementations suffer from heat dissipation problems due to the high thermal resistance mainly caused by the silicon island being surrounded by trench isolation and buried oxides.
As circuit operating currents and bias voltages have increased, thermal instability has become a significant issue for reliability of high performance SOI BJTs. Accordingly, circuit designers rely significantly upon device characterizations establishing SOAs for such devices. As is well known in the art, the conventional techniques for establishing the SOA for a power BJT is to test discrete transistors in fixed single modes which are generally described as voltage (Vbe) controlled mode and current (Ibe) controlled mode.
Referring to FIG. 1A, a typical voltage controlled mode of test for a BJT has the transistor Q connected in a common base configuration, i.e., with the base electrode grounded, and collector Vcc and emitter Vee bias voltages applied at the collector and emitter terminals, respectively. The voltage at the emitter terminal is controlled so as to provide a controllable base-emitter voltage Vbe. The collector voltage supply Vcc is also controlled so as to provide a variable collector-emitter voltage Vce.
Referring to FIG. 1B, such a voltage controlled mode, absent current limiting within the collector power supply Vcc, will bring about thermal runaway due to destructive feedback between current and temperature within the transistor Q, along with an abrupt drop in current gain (beta) at a particular base emitter voltage Vbe. Around the critical base emitter voltage Vbe (generally between 0.8 and 0.9 volts), the current within the transistor Q causes the temperature to rise. This, in turn, stimulates a further increase in the current, which induces a further temperature rise. This process repeats until the transistor fails (or the current limit established in the collector power supply Vcc is reached). The onset base emitter voltage Vbe (i.e., the voltage at which the abrupt current gain decrease occurs) and emitter current density Je for thermal runaway decrease as the collector emitter voltage Vce increases.
Referring to FIG. 2A, for the current controlled mode, the transistor Q is again connected in a common base configuration and with a controllable collector bias voltage Vcc supply. In this mode, the emitter is driven by a current source lee which is controllable to provide a variable emitter current Ie which, in turn, causes a variable base current Ib and base emitter voltage Vbe to be generated.
Referring to FIG. 2B, in the current controlled mode, as the bias current Ie is increased, thereby causing the collector current Ic to increase, the base emitter voltage Vbe will increase to a point and then decrease, thereby indicating a negative resistance in the high power region of operation. However, with no spontaneous feedback between voltage and temperature for a fixed current Ie, the transistor Q may not fail. The onset base emitter voltage Vbe and emitter current density Je for negative resistance both decrease as the collector emitter voltage Vce increases.
Referring to FIG. 3, as a result of these voltage controlled and current controlled mode tests, the safe operating areas for the transistor Q can be established based upon the thermal runaway modes (FIGS. 1A and 1B) and negative resistance modes (FIGS. 2A and 2B) as a function of the collector-emitter voltage Vce bias. For this particular example (NPN transistor with a 40 square micrometer emitter area Ae), the critical current density Je (in milliamps per square micrometer) for the thermal runaway condition is half of that for the negative resistance condition.